The backlighting of translucent plastic elements, such as push-button caps with backlit icons, with narrow light sources, such as color LEDs, is easy to control the light transmitted to the eye of a viewer because the color varies only minimally with the wall thickness of the push-button caps. Therefore, the brightness of an LED can be adjusted by varying the wall thickness of the push-button caps.
With a broadband light source such as a white LED, however, it is more complicated. With a broadband light source, both brightness and light color may vary with the wall thickness of the material covering the light source. Varying the wall thickness of the material covering the light source to produce a homogeneous illumination of the symbol on the covering can vary the color that reaches a viewer's eye. Often move to warmer white translucent materials white light through the material back shades.
FIG. 7 shows an example of the prior art. A conventional lamp 41 comprises a white LED 42 with a white translucent plastic element 43 with constant wall thickness. The plastic element 43 is in the path of the emitted white light 44 of the LED 42. During the passage of the white light 44 by the white plastic element 43, the white light 44 is shifted towards yellow as shown by the arrow 45.
FIG. 8 shows another example of a prior art lamp 51. The lamp 51 comprises a white LED light 52 and white translucent plastic element 53 having a varying thickness (in this figure, wedge-shaped). The plastic element 53 is disposed in the beam path of the emitted white light 54 of the LED 52. The LED 52 is positioned approximately equidistant from the right and left ends of the plastic element 53, such that the intensity of the emitted white light 54 is approximately the same at each end of the plastic element 53. When the emitted white light 54 passes through the white translucent plastic element 53, the white light 54 is shifted towards yellow, and does so more the thicker the wall thickness of the white translucent plastic element 53. As indicated by the different gray arrows 55a and 55b, the light (indicated by arrow 55a) emitted through the plastic element 53 at the tip of the wedge is brighter and whiter than the dimmer, yellower light (indicated by arrow 55b) emitted through the plastic element at the stump of the wedge formed by the white translucent plastic element 53, as indicated by the magnitude and shading of the arrows 55a and 55b. Due to the wedge shape, as light passes through the wedge stump more light is absorbed than light passing through the thinner wedge tip, such that the result of the light passing through has different intensities on the wedge stump and wedge tip, as indicated by the different lengths of arrows 55a and 55b. 